1. Field of the Invention
The present invention is related to a backlight module and related driving method, and more particularly, to a backlight module with dynamic open-lamp protection and related driving method.
2. Description of the Prior Art
Liquid crystal display (LCD) devices, characterized in thin appearance, low power consumption and no radiation, have been widely used in various electronic products, such as computer systems, mobile phones, and personal digital assistants (PDAs). In a prior art LCD device, brightness/contrast adjustment is performed by controlling the driving voltage/current of a light source in a backlight module of the LCD device. The contrast ratio of the LCD device can be largely improved (such as from 500:1 to 50000:1) using dynamic contrast ratio (DCR) technique. DCR technique can be implemented using an image processing system which automatically detects the image brightness of the input signal and dynamically adjusts the brightness of the backlight module accordingly. Therefore, DCR technique can reduce light leakage when displaying dark images, and can increase the contrast between bright and dark images.
Reference is made to FIG. 1 for a functional diagram of a prior art backlight module 100 with static open-lamp protection. The backlight module 100 includes a light source 150, a transformer 110 for driving the light source 150, an inverter controller 120, and an open-lamp protection circuit 130. The light source 150 may include lamps LAMP1-LAMPN coupled in parallel, and the input end of the light source 150 is coupled to the transformer 110 for receiving the driving voltage. The voltage established at the input end of the light source 150 is represented by a feedback voltage VFB, and the brightness of the lamps LAMP1-LAMPN is related to lamp currents IL1-ILN, respectively. The open-lamp protection circuit 130, coupled to the output end of the light source 150, is configured to provide a pseudo open circuit voltage VOP according to the lamp currents IL1-ILN. The inverter controller 120, having a first input end coupled to the input end of the light source 150 and a second input end coupled to the open-lamp protection circuit 130, is configured to provide a power control signal SCT by comparing the voltage levels of the feedback voltage VFB and the pseudo open circuit voltage VOP. The transformer 110 can thus adjust the driving voltage according to the power control signal SCT. When the lamps of the light source 150 function normally and the display device 1 operates under medium/low contrast mode, the feedback voltage VFB (such as 0.9V) is smaller than the pseudo open circuit voltage VOP (such as 1.5V). At this time, the inverter controller 120 outputs the power control signal SCT to the transformer 110 and the inverter controller 120 outputs the driving voltage for driving the light source 150. When an open-lamp defect (open circuit) occurs in the lamps of the light source 150, the feedback voltage VFB becomes larger than the pseudo open circuit voltage VOP. At this time, the inverter controller 120 stops outputting the power control signal SCT for turning off the transformer 110, thereby turning off the backlight module 100. When DCR function (high contrast mode) of the display device 1 is activated, the lamp currents IL1-ILN, the feedback voltage VFB of the light source 150 and the pseudo open circuit voltage VOP of the open-lamp protection circuit 130 need to be lowered in order to provide more brightness options. For example, when the feedback voltage VFB drops below 0.7V and the pseudo open circuit voltage VOP drops below 0.2V, it is determined that an open-lamp defect occurs in the light source 150. The transformer 110 is then inadequately turned off, which in turn influences the operation of the display device 1.
Reference is made to FIG. 2 for a functional diagram of a prior art backlight module 200 without open-lamp protection. The backlight module 200 includes a light source 250, a transformer 210 for driving the light source 250, and an inverter controller 220. The light source 250 may include lamps LAMP1-LAMPN coupled in parallel. The voltage established at the input end of the light source 250 is represented by a feedback voltage VFB, and the brightness of the lamps LAMP1-LAMPN is related to lamp currents IL1-ILN, respectively. The inverter controller 220 includes a first input end for receiving a constant voltage VCC and a second input end for receiving the feedback voltage VFB, thereby generating the power control signal SCT accordingly. The prior art backlight module 200 does not provide open-lamp protection, and the display device 2 can provide multiple brightness options using the small lamp currents IL1-ILN without misjudging open-lamp defects. However, if an open-lamp defect occurs in the lamps of the light source 250, the backlight module 200 cannot be turned off and the transformer 210 continues to output high-level voltages, which may cause arcing phenomenon and endanger the safety of the display device 2.